The invention relates to a method of starting and/or regenerating a cryopump and a cryopump therefor.
A cryopump or cryogenic pump is a temperature (cold) operated vacuum pump. Cryopumps having two-stage refrigerator operation are disclosed in published German patent applications DOS 26 20 880, 28 21 276 and 30 38 415. Each of these has three surface areas for adsorption or absorption of various types of gases.
The first surface area is in good thermal contact communication with the first stage of the refrigerator, which, in operation, has a substantially-constant temperature between 60.degree. and 100.degree. K., depending on the type and capacity of the refrigerator, and has a conductivity appropriate to provide a corresponding, substantially-constant temperature with only a small temperature gradient across the surface area. A metal having appropriate thermal conductivity is, therefore, generally used for its structure which may also serve as a baffle that protects other, lower-temperature refrigerator-pumping surfaces from incident heat radiation. The main purpose of the first surface area is, however, to adsorb water vapor and carbon dioxide by cryocondensation. Cryocondensation occurs when gases impinge on a sufficiently-cold homogeneous surface and condense on it into the liquid or solid phase. The binding forces are of a physical nature, and the binding energy corresponds to the heat of vaporization.
The second surface area has similar good thermal contact communication and conductivity, but with the second stage of the refrigerator to be at its colder temperature and, therefore, is likewise generally a metal surface. It is intended for removing argon, carbon monoxide, methane and halogenated hydrocarbons, and, perhaps, hydrogen, for example, by cryocondensation and cryotrapping. Cryotrapping is a process in which lower-boiling and, therefore, more-difficultly condensable gases impinge on a sufficiently-cold surface simultaneously with more-readily condensable gases and the more-difficultly condensable gas are absorbed in the steadily growing condensate layer of the more-readily condensable gas.
The third surface area is also at the temperature of the second stage of the refrigerator (or at a correspondingly lower temperature in the case of a three-stage refrigerator) and is coated with an adsorbent such as activated charcoal or the like. It is intended essentially for removing lighter gases such as hydrogen, helium and neon by cryosorption. Cryosorption occurs when gases impinge on an adsorbent-coated and, thus, heterogeneous surface and are bound by unsaturated residual valences of the interfacial atoms of the surface. The third surface area is, therefore, arranged in such a way that gases can reach it only by "detours" or a circuitous route which the lighter gases can traverse by diffusion but heavier gases practically cannot so that they condense on the more-readily-reachable cryocondensation and cryotrapping surfaces. Premature contamination of the adsorbent with heavy gases is thus prevented to preserve its surface-atom-dependnet pumping activity for the lighter gases for a longer time.
Nevertheless, the pumping capacity of the cryopumps disclosed in the above patent publications and other known cryopumps for helium is quite poor because of the very low boiling point of helium. To improve the helium pumping capacity, therefore, cryopumps with a three-stage refrigerator or with a two-stage refrigerator and a Joule-Thompson stage have been proposed to achieve lower temperatures for the cryosorption pumping surfaces. However, these approaches are extremely complicated technically and, therefore, expensive.